In many ways the present state of optical computing may be compared to the period of the early 1950's when electronic computers were making the transition from analog to digital systems. The development of analog systems on an integrated optical circuit sometimes hereinafter referred to as "IOC" has by now achieved a certain degree of maturity. Bulk processing devices which simulate the arithmetic operations of multiplication and addition digitally are just now beginning to emerge. The correpsonding devices should have the size and speed capabilities of VLSI circuitry. The application of these devices to high speed residue arithmetic operations is also possible.
An electronic digital computer may be viewed in its most elementary form as an aggregate of simple logic elements such as NAND's and NOR's. When properly interconnected and programmed, these elements combine together to produce the variety of logic functions required for modern computing applications. Simple IOC logic elements are also known in the art. The devices currently in use include directional-couplers, waveguide interferometers, photoconducting overlays, electro-absorption modulators, cut-off switches, bistable devices, pyroelectric cyrstals, cleaved-coupled-cavity (C.sup.3) semiconductor lasers, cross channel waveguides with electrooptic mirror electrodes or electrooptic Bragg diffraction electrodes, and crossed channel two-mode interference switches. Each of these devices is relatively large. Their dimensions are usually in the hundreds of microns or more, as opposed to tens of microns or less for VSLI components. Also, light traveling sequentially from one logic element to the next changes amplitude far more quickly than voltage is dropped in a corresponding sequence of electrical switching elements. In a complex IOC network, the digital input-output characteristics of a system based on simple optical logic elements quickly deteriorates making the system unuseable for high component density applications. In the case of the photoconducting overlays, if light does not travel through the channels sequentially, it must be used to activate a number of switches sequentially, which also results in a deterioration of the digital characteristics of the system. The alternatives for all of the simple logic elements are to introduce light repeaters for every few elements or to use thousands of independent light channels. In the light channel approach, the output of each logic element may be converted to an electrical signal and used to switch a corresponding output channel, fed directly from the original source. Both alternatives clearly reduce the packing density of such devices, thus rendering them unacceptably large for commercial use. Optical threshold logic elements in accordance with the present invention provide a solution to this dilemma.